13. Transparent
Clear – you can see right through it when it is sliced
thin
Called “gemmy” because it is desirable for gemstones
Celestite, quartz (rock crystal), selenite
14. Translucent
See shapes and shadows through it when it is
sliced thin
Chemical impurities can cause the minerals to be
cloudy
Calcite, quartz, sphalerite
15. Opaque
Can’t see light through it at all when it is sliced
thin
Rarely used for gemstones
Metals, gypsum
16. Ulexite
Ulexite appears to be
opaque when you look at it
from the side, but when you
look at it from the top, it is
transparent. This is because
light is able to travel up the
perfectly straight columnar
crystals. It’s sometimes
called “T.V. rock” because
of this special property.